Television broadcaters supply television signals to an antenna for broadcasting television signals to the public. Very often, the income of a television broadcaster depends upon fees charged to advertisers who desire to reach the public with their messages. In order to maximize his income, a broadcaster attempts to maximize the number of members of the public capable of receiving his signal. He may do this by increasing the gain of his antenna and the height of the antenna above the surrounding terrain so as to increase the line-of-sight distance to the horizon to reach more distant viewers. He may also choose to couple the maximum possible television signal power to the antenna, and to keep the broadcasting station operating under all conditions. One way to increase the signal power coupled to the antenna is to install a single large transmitter or power amplifier and its ancillary equipment, and to couple this single transmitter to the antenna. This has the disadvantages that routine maintenance to the transmitter may require down-time (time in which the transmitter is not operating), and similarly a failure of the single transmitter renders the broadcasting station inoperative. It is also known to use two transmitters or high power amplifiers in parallel and to couple the power from each of the transmitters to the antenna. This has the advantage of improved reliability, in that operation at reduced power continues if one of the transmitters fails, and further has the advantage that a given output power can be achieved by the use of a plurality of inexpensive low power output stages rather than by means of a single expensive high power unit.
It is often necessary to operate a transmitter into a dummy load in order to perform tests or for alignment, and signal switching is therefore necessary. Motor-driven contactors (relays) may be used for switching in order to couple one of two transmitters to an antenna, while the other transmitter is maintained in a standby condition in case the on-line (operating) transmitter should fail or in the event that routine maintenance is required. If has been found, however, that it is disadvantageous to use contactors for switching the output of the transmitter from an antenna to a load or from a load to an antenna while the transmitter is in full operation, because of various problems related to arcing at the contactor and changes in transmission line impedance while the contactors are operating, which may cause high voltages to appear at the output of the transmitter. When contactors are used to switch the output of a transmitter, it is common to shut down or inactive the transmitter by removing the energizing voltage therefrom before the contactor is operated to switch the output circuit. This mode of operation, however, is very disadvantageous to a broadcaster who does not wish to have any broadcast down-time, since it may take several seconds to shut down the operating transmitter, operate the motor drive to the motor contactor, and start up the substitute transmitter.
In order to allow switching of the transmitter output while energized, phase-shift controlled switching has been developed. Generally speaking, an arrangement for such switching includes directional or hybrid coupling circuits for combining the outputs of two transmitters and for coupling them by way of two paths to a further hybrid having one output coupled to the antenna and another coupled to a dummy load. Each of the two paths includes a controllable phase shifter, each phase shifter of which includes a further hybrid circuit and controllable reactive terminations. Such an arrangement allows both transmitters to be operated simultaneously and allows the signals to be switched between the antenna and the load without the switching of contacts and without deenergizing the transmitters. Instead, the switching is accomplished by selective control of the reactive terminations associated with the circuit. Control of the reactances causes the signals arriving at the antenna or at the load by the two paths to be either in-phase and therefore add, or to be out of phase and therefore cancel. The aforementioned phase shift controlled switching eliminates the problems associated with contactors, but may have undesirable characteristics if motorized vacuum capacitors are used for the reactive terminations. Ordinary motorized vacuum capacitors are relatively slow in operation, because of the screw drives required to maintain vacuum integrity. Thus, the change in phase which is required in order to effect switching may take several seconds, notwithstanding that the transmitters need not be deenergized during the switching. Also, the motor drives and motor drive control circuits for vacuum capacitors must be made with great precision, for a slight error in the resting position of the motor driven capacitor at either extreme of operation may result in incorrect values of terminating reactance and corresponding incorrect phase shift. Since switching of a signal away from an output port is accomplished by cancellation of two oppositely-phased signals, slightly incorrect phase in a path to the cancellation point may result in failure to cancel, which in turn results in undesired crosstalk (insufficient isolation) between signals at the loads. Furthermore, vacuum capacitors are expensive, subject to a failure mode in which the vacuum seal is lost, and may not be available in the combinations of capacitance values and power-handling capability which are desired for broadcast transmitter use. Vacuum capacitors may also have power-dissipation problems resulting from the high current flows at high power levels, because the vacuum prevents convection cooling of the capacitor plates.
It is possible to substitute a coaxial variable inductance-capacitance circuit for the vacuum capacitor of the prior art. Such a coaxial variable inductance-capacitance circuit includes a series coaxial capacitance formed by a hollow inner conductor having a gap which is centered in a coaxial inner conductor. An insulated conductive slug is located within the hollow center conductor and is moveable between a position straddling the gap and a position remote from the gap for varying the capacitance across the gap. A series inductance trims the capacitance. This arrangement is very advantageous by comparison with the vacuum capacitor, because the rate of change of reactance at positions in which the conductive slug straddles the gap and is remote from the gap is essentially zero, and therefore the reactance is substantially independent of the precise position of the conductive slug. This arrangement has the further advantage that the actuators for the capacitors do not require precision alignment and maintenance, and convection or forced-air cooling may be used. While this switching arrangement is very advantageous, its size may present packaging problems at frequencies corresponding to low VHF television channels (54 to 88 MHz), because four hybrid or directional couplers and two independently actuated sets of reactive terminations are required, each set including a pair of simultaneously actuated coaxial inductance-capacitance circuits.
It is desirable to provide contactless switching among sources and loads by means of a simplified apparatus, while maintaining the advantages of coaxial inductance-capacitance circuits.